1. Field of the Invention
The present invention generally relates to overvoltage protection circuits, and more particularly to voltage surge protection circuitry which protects equipment connected to communication lines when they are hit by lightning.
2. Discussion of the Relevant Art
Many circuit arrangements have been developed over the years to protect communications and other equipment from being destroyed when lightning strikes, which causes tremendous overvoltage surges on the communication transmission lines. Equipment connected to the communication lines are not designed to withstand these high voltage surges and, in order to protect them, it is necessary to provide very rapid overvoltage protection circuitry that does not dissipate significant amounts of power. Moreover, the protection circuit is not to be activated or tripped when the ringer voltage of 120 volts is applied to the line. In addition, the electrostatic capacitance should be no higher than 200 pico farads (pf), since the transmission loss standards specify that the signal line impedance with respect to ground should be greater than 4,000 ohms to enable the maximum frequency component of the transmitted signal to be carried with a limited dissipation assuming a digital transmission speed of 200 kb per second. The protection circuit should also be capable of handling a lightning surge currents in excess of 100 Ampere with current wave rising to a peek value in ten microseconds and exponentially falling to one-half the peek current value in 1,000 microseconds. Thus, in addition to clamping the voltage on the transmission or communication lines to a safe value it is also necessary that the protection circuit absorb large amounts of current for short periods of time. The need to carry large currents requires that the semiconductor device has a relatively large surface area, which generally increases the amount of capacitance that the device has. Placing increased capacity across the communication line prohibits the higher frequency components of the communication systems to be limited.
The patent to Satoh, et al. utilizes a thyristor, triac, and silicon symmetrical switch (SSS) connected in series with a voltage limited bi-directional zener or thyristor, varistor, metal oxide varistor (MOV) or a Transorb unit connected in series from one side of the line to ground. However, the bi-directional zener, although generally fast operating, provides capacitance across the line which is greater than desired.
The present invention overcomes the short comings of the prior art by providing a circuit arrangement that reduces capacitance loading on the communication line. The breakover voltage is determined by the thyristor device and not the series diodes. The insulation resistance of the circuit arrangement increases with the use of the diodes and the leakage current through the combination decreases. The response time is not altered by using the series diodes and the overshoot with fast rising transients are virtually non existent.
Therefore, it is an object of the present invention to provide an improved overvoltage protection circuit.
It is a further object of the present invention to provide an improved overvoltage protection circuit which has a short response time.
Still another object of the present invention is to provide a solid state protector circuit which has fewer components and which may be used on high frequency transmission systems.
Yet another object of the present invention is to provide a circuit arrangement which utilizes relatively inexpensive circuit components.